The present invention relates in general to aligning the frequency response of a variable bandpass filter in a radio receiver, and more specifically to obtaining a reference frequency in a multiband radio receiver for use in an alignment procedure for a variable bandpass filter.
Radio wave receivers (e.g., AM and FM radio, TV and mobile transceivers) simultaneously sense many different broadcast signals at their antennas. A desired signal must be separated from others in the broadcast band in order to present it to the user of a receiver. Each separate signal occupies a unique frequency range. Thus, a desired signal may, at least partially, be separated from other signals by connecting an antenna to a bandpass filter which attenuates all frequencies other than those in the desired frequency range. In order to selectively receive different frequency ranges (i.e., stations or channels), the bandpass filter comprises a variable radio-frequency (RF) filter. One such variable filter includes voltage variable capacitors known as varactor diodes, as well as fixed capacitors and fixed inductors in a bandpass configuration wherein a voltage applied to the varactor diodes controls the resonant frequency (i.e., center frequency) of the filter. When tuning in a radio-wave broadcast signal at a particular carrier frequency, a control voltage applied to the varactors causes the center resonant frequency of the RF filter to coincide with that carrier frequency. Radio-wave signals outside the passband of the RF filter are attenuated, thus improving the selectivity of the receiver. However, the width of the passband of a particular RF filter is greater than the bandwidth of a particular broadcast signal so that the RF filter output may include several broadcast signals. Further selection of the desired broadcast signal is performed in an intermediate frequency section of the receiver as is known in the art.
Due to variability of characteristics of individual capacitors, inductors, and varactors used in constructing a tunable RF filter, the filter response must be aligned (i.e., calibrated by frequency) for each individual receiver. In order to find the precise control voltage to be applied to a varactor in a filter to obtain the desired frequency response, a known reference frequency is typically applied to the input of the variable filter during the alignment procedure. The filter control voltage is then varied throughout its range in order to maximize the magnitude of the filter output. At maximum output, the frequency response of the variable filter is assumed to be properly aligned. Since the relationship of the filter control voltage to the filter center frequency is not linear, the variable RF filter must be aligned for at least two frequencies in order to adequately determine that relationship. Thus, at least two reference frequencies are needed (e.g., the lowest and highest frequencies in a desired broadcast band).
Alignment is preferably done automatically without use of external devices in order to save manufacturing time, labor, and cost. Prior art receivers with automatic alignment have obtained a reference frequency from either a variable oscillator already present in the radio receiver or an auxiliary source dedicated to providing the reference frequency.
The use of extra hardware is undesirable, especially at higher frequencies, such as within the FM band, television band, and mobile communication band because of high cost. On the other hand, if the variable oscillator present in the superheterodyne tuner is used to provide the reference frequencies, then the oscillator is not available to perform its usual task of mixing the radio-frequency signal (reference signal) to a lower intermediate frequency. As a result, the magnitude of the filter output must be measured and maximized at the RF frequencies rather than at the intermediate frequencies. Thus, circuits normally already present for measuring the strength of intermediate-frequency signals cannot be used. Furthermore, detection of the level of an RF signal can only be done inaccurately and subject to many errors unless expensive and complicated extra hardware is employed.
In commonly assigned and co-pending application Ser. No. 582,579 entitled "RF FILTER ALIGNMENT USING DIGITAL PROCESSOR CLOCK", filed Sept. 14, 1990, now U.S. Pat. No. 5,101,509 issued Mar. 31, 1992 reference RF frequencies are provided by the clock signal of a digital processor. Several reference frequencies within a receiving band of a receiver are obtained from the clock signal by employing harmonic frequencies. However, alignment is not possible at reference frequencies between harmonic frequencies. Furthermore, if the difference between the fundamental clock frequency and the receiving band of the receiver is too great, then the magnitude of the harmonic frequencies from the clock signal may be inadequate to provide a reference frequency within the receiving band.